We also do Nicole's second option on a couple of our forms.
But instead of storing the file on another file sharing platform, we store it right in Workfront. We have a documentation project to store that sort of thing as well as SOPs, we make sure to share the file with any groups that have access to submit to that queue, and use the same URL field to enter the URL for that document.
But I can also read your question another way... are you asking if a document can be uploaded while the user is filling in the custom form on a request queue? The document upload area can be turned on or off in the Queue Details for each request queue.